Wastewater treatment plants use a lot of energy. Recent research shows that wastewater sludge can produce enough energy to operate the plant. And sewage is an infinitely renewable resource.
Energy from sludge can replace electricity from fossil fuels. Sewage sludge as renewable energy stands to save the towns and cities that operate wastewater treatment plants a lot of money. Potentially, they could even make more clean energy than they need and sell the excess.
Several wastewater energy technologies are in some stage of development. Some work now. Others might take a long time to become scalable. Here are a few possibilities:
Microbial fuel cells
Microbial fuel cells work like an ordinary battery in a sense.
In a battery, the oxidation reaction takes place in one electrode, the anode. That is, a chemical process produces electrons, which travel to the other electrode, the cathode. A reduction reaction occurs in the cathode. That is, a chemical process reabsorbs the electrons. Some kind of load intercepts this current and uses it for power.
In a microbial fuel cell, anaerobic bacteria live in the anode. They require a constant food supply, such as sewage sludge. They can have an abundant supply of sludge. In the process, they give off electrons. These electrons travel to the cathode, an aerobic chamber. This current can power some kind of load, just like a battery.
It’s easy enough to make a microbial fuel cell about the size of a golf ball. Such small cells don’t produce much current. Sooner or later, scientists are confident that they can scale up microbial fuel cells to supply energy for a wastewater treatment plant. It will probably take decades for them to succeed.
Methane from sludge
Hydrogen fuel cells likewise promise to provide clean energy. We don’t have to wait so long for them to work, either.
Sewage treatment plants have several ways to treat sludge, including anaerobic digestion. Anaerobic bacteria eat the sludge and give off biogas, including methane and hydrogen sulfide. In many sewage plants, these gases present a disposal problem. Instead, the hydrogen in the gasses they emit can make fuel cells work.
The deadly hydrogen sulfide can corrode the equipment without special handling. Any system to get energy from sludge must also deliver hydrogen at specific pressure levels.
Spencer Turbine Company has installed just such a system at the Hunts Point Water Pollution Control facility in New York City. Plants in Xiangyang, China and Aarhus, Denmark likewise run on biogas from sewage sludge. The one in Aarhus generates 150% more energy than it uses. The excess pumps clean drinking water to city residents and businesses.
Anaerobic co-digestion of fats, oils, and grease
You should never, ever pour fats, oils, and grease (FOG) down the drain or toilet. It’s a problem the wastewater treatment system must wrestle with. But like many problems, FOG can become a resource. It can enhance making electricity from sludge.
A team of scientists at Oregon State University has begun to study how to harvest methane from FOG using co-digestion. Co-digestion means mixing two different substrates, or places where bacteria live. In this case, they add some FOG to ordinary anaerobic digestion. The combination releases more methane than the sludge alone.
FOG co-digestion is more complicated than it might seem at first. After all, more than one species of bacteria lives in wastewater sludge. The chemical makeup of the FOG also varies.
Once the scientists devise a system that can work dependably anywhere, the methane can make plants net producers of energy. They will be able to sell the electricity they don’t.
Universities invent many useful technologies that, alas, remain hidden. This one already works at industrial scale now. The sewage plant in Gresham, Oregon started using FOG co-digestion in 2012. Three years later, it was producing all the energy it needed to operate. It saved the town about half a million dollars in electricity costs.
Disinfection of sewage sludge without chlorine of UV light
Part of the sewage treatment process requires killing all the disease-carrying germs. Most plants use chlorine. It kills germs all right, but it’s not especially good for humans. Others use ultraviolet light. It makes their electric bills skyrocket.
A company in San Leandro, California called Pasteurization Technology Group (since renamed PTG Water & Energy) has found an alternative. It uses cogeneration. That is, it produces both electricity and useful heat from the same sludge.
As the name implies, the process entails pasteurizing the sewage. It uses the heat to increase the water temperature to 180º F. And the heat comes from sewage digesting equipment wastewater treatment plants already have.
Besides municipal water treatment plants, the technology works for industry. PTG claims its process reduced a Los Angeles craft brewery’s power costs by 80%.
When I looked at the company website, I noticed that the latest “recent news” is dated in May 2018. I found the company in a news story as recent as June 2019.
I can’t tell if PTG will make it financially. Many companies invent promising new technologies. Then they struggle to build viable businesses.
Geothermal energy from sewage mains
It’s actually possible to make wastewater energy even before sewage gets to the treatment plant. Wastewater is warmer than clean water. That heat can heat and cool commercial buildings. The process works like any geothermal system.
Geothermal sewage energy requires a heat pump connected to the municipal sewage main. It also requires a way to prevent solids from blocking the pump. A Chinese company called Jin Da Di invented such a device and licensed it to Nova Thermal Energy in Philadelphia.
According to a 2012 press release, the company had a public-relations problem. It had trouble persuading squeamish businesses that running sewage from an existing main through a heat pump would not harm their buildings. I have found nothing more recent to indicate that anyone in the US makes clean energy from sewage this way.
So here are five different technologies to derive clean, renewable energy from wastewater. Some appear close to large-scale operation. Some may not make it. One has apparently failed, but the technology exists for any company that wants to obtain a license.
Wastewater energy recovery is a reality with great opportunity for growth.
A new way to clean up dirty water, and generate power in the process / Ariel Schwartz, Fast Company. February 6, 2012
Here’s how clean energy is coming from dirty water / Kayla Matthews, Blue & Green Tomorrow. March 30, 2020
Sewage, geothermal energy, and the yuck factor / David Guion, Sustaining Our World. April 26, 2012
We’re turning dirty water into clean energy / Oregon State University College of Engineering. August 16, 2019